Degenerative electrical and electromechanical failures can be foretold by performance variations of individual electrically driven loads, particularly during start-up. Systems for monitoring such performance variations gather and analyze electrical data to detect potential electrical and electromechanical problems before the occurrence of more significant failures.
Both electrical and electromechanical problems, particularly those related to electric motors and their drive circuitry, often develop gradually and can be detected well in advance of outright failure. For example, connection points can loosen, contacts can become pitted or burned, and bearings or other moving parts can wear or partially break. These problems can be detected before they become serious enough to cause permanent damage, system shutdowns, or other significant forms of failure.
Some instruments for detecting electrical and mechanical problems require the motors to be analyzed off-line to exercise more systematic control over the motors and to exclude other electrical influences. Other instruments allow the motors to be continuously monitored on-line, but these are generally mounted with the motors and add considerable cost. Portable instruments, such as infrared detectors and power analyzers, can be used to take periodic readings of motor and drive circuit efficiencies but are time-consuming to set up and operate.
An on-line diagnostic system for analyzing the performance of a polyphase motor is described in U.S. Pat. No. 5,519,300 to Leon et al. Demodulated current and voltage measurements are taken from each of three phases to monitor electrical characteristics useful for detecting problems with the motor. These characteristics include total real power, real power per phase, total reactive power, reactive power per phase, total apparent power, apparent power per phase, overall power factor, power factor per phase, electrical impedance per phase, and electrical balance.
Leon et al.""s diagnostic system can be used for one-time analysis or for historical trending by monitoring the motor at different times. However, the expense of this system, which requires sensors, signal conditioners, analog-to-digital converters, and a computer, can be too great for the limited purpose of monitoring a single motor. Moving the system to individually monitor more than one motor would be time-consuming and would interfere with the accumulation of trending information.
My system, which I refer to as an OMNI-BUSS(trademark) system, monitors a plurality of electrical loads, particularly drive motors, from within a common electrical supply circuit. The electrical loads can be monitored from one location with a single array of sensors. Trending information can be accumulated for all of the loads to note changes or patterns associated with deteriorating electrical or electromechanical conditions.
One version of my preferred system senses electrical characteristics of a common electrical circuit that supplies a number of different electrical loads. When the electrical loads undergo a change in load condition, such as during start-up or during a significant change in speed, information is transmitted concerning the identities of the electrical loads. The transmitted information links the sensed electrical characteristics of the common electrical circuit with the individual electrical loads. Performance variations of the electrical loads are monitored by noting changes in the sensed electrical characteristics of the common electrical circuit.
The electrical characteristics (e.g., current and voltage) of the common electrical circuit are preferably stored, and limited intervals of the stored characteristics are linked with the individual electrical loads. The stored intervals preferably include electrical characteristics measured both just prior to and during the changes in load condition. The electrical characteristics stored just prior to the changes in load condition provide a background reference or baseline against which the electrical characteristics accompanying the changes in load condition can be measured.
The load-identifying transmissions can be broadcast through the air or conveyed through a data bus or the electrical circuit itself to a central receiving site. The changes in load condition that trigger the load-identifying transmissions can be detected from sites connected to branch load circuits containing the electrical loads. Only a minimum of sensing is required in the branch load circuits to identify the changes in load condition. Start-ups, for example, are detected by the onset of current flows in branch load circuits.
The sensing of the common electrical circuit preferably includes sensing both current and voltage values in each phase of multi-phase electrical systems. This information can be processed to determine other electrical characteristics such as apparent power, real power, and power factor for each phase. Comparisons can be made between phases and between stored intervals associated with the same loads to detect abnormalities or progressive changes indicative of electrical or electromechanical problems in the branch load circuits.
A data acquisition system for practicing my invention preferably includes sensors that measure electrical characteristics of the common electrical circuit. Transmitters operatively coupled to the branch load circuits emit signals identifying electrical loads as they undergo changes in load condition. A data processor receives information (a) from the sensors monitoring electrical characteristics of the common electrical circuit and (b) from the transmitters monitoring changes in load condition. The data processor associates limited intervals of the sensed electrical characteristics with the electrical loads that undergo changes in load condition within the intervals.
Preferably, a recorder within the data processor continuously stores the sensed electrical characteristics. For example, a circular buffer can be used to temporarily store the sensed electrical characteristics. Stored intervals recorded just prior to and during the changes in load condition can be written to a more permanent data storage file for each load. Conventional algorithms can be used to analyze this data for trends or for more immediate indications of electrical or electromechanical problems. The analysis can be performed contemporaneous with the data acquisition or on a deferred schedule.
The sensors within the common electrical circuit can be further arranged to distinguish between the contributions of multiple power sources. For example, current outputs from a pair of generators can be compared each time a change in load condition triggers the further processing of information from the sensors. Similar to the analysis applied to the individual loads, information concerning the performance of the generators or other power sources can be evaluated to note deteriorating or out-of-tolerance conditions. Similar information can also be derived concerning the common electrical circuit itself.